Implantable medical devices, such as pacing apparatus, generally include detection algorithms to detect various types of heart conditions, e.g., ventricular fibrillation (VF), ventricular tachycardia (VT), supra-ventricular tachyarrhythmia (SVT), etc., that require employment of therapy. For example, detection algorithms may be based on heart rate, or in other words R—R intervals, alone or in combination with other additional criteria, such as sudden onset criteria, rate stability criteria, and QRS width. For example, dual chamber pacemaker-cardioverter-defibrillators (PCDs) may be able to differentiate supra-ventricular tachyarrhythmias from ventricular tachycardias using an algorithm that processes atrial and/or ventricular EGM signals. In many cases, to increase detection accuracy and/or to distinguish various types of arrhythmias, such as various types of ventricular tachyarrhythmias, devices may include a type of hemodynamic sensor, e.g., an intra-cardial blood pressure sensor and/or a flow sensor.
A common approach to programming PCDs uses the specification of discrete heart rate zones (e.g., R—R interval zones) for ventricular fibrillation (VF) and various types of ventricular tachycardia (VT). Each rate zone for the detection algorithms, for example, may be defined using different heart rates and, correspondingly, different R—R interval values. For example, a PCD may have three different heart rate zones: VT, fast VT, and VF. Each of these rate zones, e.g., VT, fast VT, and VF, may be treated by a separately defined therapy.
Moreover, therapy for any particular type of arrhythmia may be delayed until a preprogrammed number of consecutive counts or intervals are obtained in which the heart rate detected is within a particular rate zone. The number of intervals detected (NID) (e.g., the number of R—R intervals detected in a particular rate zone) must reach a set value before therapy will be delivered. This threshold may differ for each identified arrhythmia, e.g., may be programmable in wide ranges separately for VF, fast VT, and VT.
Generally, systems that depend upon the aforementioned heart rate criteria are capable of discriminating tachycardia in a greater or lesser degree from normal heart rate. However, such systems may have difficulty discriminating hemodynamically stable tachycardia from hemodynamically unstable tachycardia.
Hemodynamically unstable tachycardia is generally accompanied by a decrease in the mean blood pressure. Hemodynamically unstable tachycardia may provoke syncope due to such a systemic blood pressure drop, which may be sudden or develop successively. Hemodynamically stable tachycardia, on the other hand, generally is not accompanied by a systemic blood pressure drop, and therefore, usually causes some kind of palpitations but not syncope. In clinical practice, hemodynamically stable tachycardia may be successfully treated by anti-tachycardia pacing, e.g., extra stimuli or bursts of stimuli. Such hemodynamically stable tachycardia is usually slow, but if not terminated, it may accelerate and become hemodynamically unstable.
For pacing devices, such as PCDs, with programmed heart rate zones for detection of various types of tachyarrhythmias, determining the type of and the time for delivery of therapy for hemodynamically stable and/or unstable tachycardia may be problematic. For example, a higher NID threshold may be desirable to increase the specificity for detection of sustained tachycardia. However, this also increases the time between tachyarrhythmia onset and the delivery of therapy. Therefore, although desirable for detection of hemodynamically stable tachyarrhythmias, such a higher NID threshold may not be able to detect hemodynamically unstable tachycardia as quickly as would be desired. Further, a lower NID threshold may be convenient for detection of hemodynamically unstable tachycardia, but undesirable for detection of hemodynamically stable tachyarrhythmias, leading to therapy that may inadvertently be delivered too early when the tachycardia is hemodynamically stable and unsustained.
Table I below lists U.S. Patents relating to detection of various types of tachyarrhythmias.
TABLE IU.S. Pat. No.InventorIssue Date5,129,394MehraJul. 14, 19925,257,621Bardy et al.Nov. 2, 19935,342,404Alt et al.Aug. 30, 19945,370,667AltDec. 6, 19945,403,355AltApr. 4, 19955,431,685AltJul. 11, 19955,458,622AltOct. 17, 1995
All references listed in Table I, and elsewhere herein, are incorporated by reference in their respective entireties. As those of ordinary skill in the art will readily appreciate upon reading the Summary of the Invention, Detailed Description of the Embodiments, and claims set forth below, at least some of the devices and methods disclosed in the references of Table I and elsewhere herein may be modified advantageously by using the teachings of the present invention. However, the listing of any such references in Table I, or elsewhere herein, is by no means an indication that such references are prior art to the present invention.